A prefeeder may be designed to handle blank sheets. The blank sheets are typically corrugated material. The prefeeder receives a stack of blank sheets, divides the stack into blocks, and feeds the blocks into a finishing machine in an intermittent shingled stream. Particularly, a block pusher prefeeder may receive the stack of blank sheets, lift the stack up, divide the stack into measured blocks, and then feed the sheets off the bottom of the block under a vertical stop in a continuous shingled stream for delivery into the finishing machine hopper.
With current block pusher technology, a stack of flat sheet stock enters the block pusher prefeeder. The lead edge of the stack is registered against a vertical stop, such as a backstop. The block pusher plate resides behind and to the top of the stack. When there is a call for another block of sheets, the stack rises, such that the stack is between the backstop and the block pusher plate. The block pusher plate then moves forward to push off a block of sheets from the top of the stack. In the standard configuration, the bottom of the block pusher plate is aligned with the top of the backstop, so as to produce a horizontal plane. This horizontal plane defines the separation point in the stack, wherein the sheet above the plane is the bottom sheet of the block and the sheet below the plane is the top sheet of the stack.
When there is down warp, the leading edge of the stack is lower than the trail edge of the stack. As a result, when the block pusher plate moves forward to deliver a block of sheets, the block pusher plate stalls due to the sheets that are captured/jammed between the block pusher plate and the backstop. When there is up warp, the leading edge of the stack is higher than the trail edge of the stack. When the block pusher plate moves forward to deliver a block of sheets, trailing sheets (i.e., sheets that are not aligned with the block or the stack) result.
Current block pusher prefeeders allow the operator to select a warp mode which lifts the block pusher plate. Elevating the bottom of the block pusher plate relative to the backstop allows the block pusher plate to convey forward and push a down warped block of sheets successfully off the stack.
Warp mode cannot be enabled permanently due to the potential for a trailing sheet condition when running flat, or non-warped, sheets. When the bottom of the block pusher plate and the top of the backstop are not correctly aligned in elevation (i.e., the bottom of the block pusher plate is above the top of the backstop), a scenario arises when running flat sheets where the bottom sheet(s) of the block, or the top sheet(s) of the stack, begin to move, but then stall and are no longer aligned with the block or the stack. This may causes issues with the manufacturing line efficiency.
With the selector switch for warp mode at the operator station, the operator is required to make the decision regarding when to use the warp mode and when to disable warp mode. Upon visual inspection of a stack, the operator can select a mode to allow the prefeeder to handle warp or select a mode where the prefeeder handles no warp. Use of a selector switch results in an increased risk for human error. For example, the operator may enable warp mode at times when warp mode is undesirable, thereby causing trailing sheets to occur. Similarly, the operator may disable warp mode at times when warp mode is desirable. Thus, the block pusher plate may stall against the back of the stack due to down warp. As an additional example, the operator may enable warp mode where warp mode is desirable (i.e., the stack contains warped sheets). However, the sheets at the bottom of the stack may be pressed flat due to the weight of the stack. That is, the amount of warp may diminish from the top of the stack to the bottom of the stack, and therefore, with warp mode enabled, trailing sheets may be present in the last few block pushes of the stack. Thus, to have an efficient operation, the operator must always be cognizant of whether warp is present in the stack and select the appropriate mode.
Thus, there is a need in the art for apparatus and methods for prefeeders with automatic warp compensation. There is a further need in the art for apparatus and methods for block pusher prefeeders with automatic warped board compensation.